This invention relates to a system and apparatus for pumping liquids according to a predetermined volumetric ratio, and particularly to a system for the pumping of viscous liquids at predetermined fluid flow ratios.
The pumping of liquids according to predetermined and relatively precise ratios has long been a requirement in industry, particularly in fields where two or more liquid components are blended according to predetermined ratio requirements, such as might be the case in the proper mixing of a paint colorant component with a paint base material. In recent years the popularity of epoxies and other materials having an active catalyst component and a base component, each of which is inert when separate but highly reactive when mixed together, has imposed new requirements on proportioning systems. Such materials require careful separation of the individual components to avoid premature curing and other adverse effects prior to the time when they are to be used. Such components are typically completely separately handled by physically separate and individual pumping systems and are delivered to a common delivery point where they are practically simultaneously mixed together and applied to the finished product. Fast curing times require fast application of the mixed material, and the reactive nature of the mixed material requires transfer of the individual components in separate form to as near the point of application as possible.
A common approach to the proportioning of such liquids has been by the use of metering cylinders sized to the proper ratio requirements, and by common coupling the pistons in such metering cylinders to a single drive source, wherein reciprocation of the drive source causes identical reciprocation of the respective metering pistons, the ultimate liquid ratio being delivered thereby being dependent upon difference in internal volume of the metering cylinders.
Another approach to proportional metering utilizing a single drive source is disclosed in U.S. Pat. No. 3,967,634, issued July 6, 1976 and owned by the assignee of the present invention. This patent discloses a reciprocating drive motor coupled to a plurality of metering cylinders through variable lever arms, wherein the stroke of the drive source causes a measured and predetermined stroke of each of the metering cylinders thereby providing different volumetric flow rates depending upon stroke adjustment.
Another approach to metering liquids is disclosed in U.S. Pat. No. 3,107,034, issued Oct. 15, 1963, wherein reciprocating hydraulic pumps are driven by pressurized hydraulic oil provided by an electric motor and pump combination. The pressurized hydraulic oil is selectively valved into each of the hydraulic pumps, thereby controlling the speed of reciprocation of the respective pumps and consequently controlling the rate of flow of the pumped material. Such a system requires the use of bypass valves and other safeguards to prevent burnout of the electric drive motor under conditions wherein the pumps are in a "stalled" delivery mode, such as where back pressure develops in the flow lines, which back pressure is developed all the way back to the electric drive motor, which inherently attempts to provide hydraulic oil at a predetermined flow rate.
U.S. Pat. Nos. 4,019,652 and 4,170,319 issued Apr. 26, 1977 and Oct. 9, 1979 respectively, disclose fluid ratio delivery system utilizing pressurized accumulators for storing the respective fluid components, and appropriate valving into a common mixing chamber wherein the characteristics of the fluid components, the size of the input ports to the mixing chamber, and the predetermined fluid pressures are all selected so that the ratio of the flow rates by weight of the liquids at the input ports is constant. Such a system may be utilized for mixing a shot or small predetermined volume of at least two fluid components on an intermittent basis.
In any liquid proportioning and pumping system wherein reactive material components are mixed and applied, it is desirable to maintain physical separation between individual components prior to mixing. Further, in any system wherein such components are intermittently applied, as by way of a paint spray gun, it is necessary that the system accommodate the "blocked pressure" condition wherein the spray gun or applicator is shut off. In this situation back pressure is developed within the system which must be relieved or compensated for so that the motive driving force does not become damaged under a "stall" condition. Obviously, when electric motors or other similar motive drive forces are used, electrical disconnecting circuits must be provided to shut off the motor under "stall" conditions. Since air-operated drive motors have inherent ability to operate under "stall" conditions it is desirable to use such motors in applications of this type where possible.